The wireless power transmission technique has been widely studied and developed since the Massachusetts Institute of Technology (MIT) conducted a demonstration experiment of wireless power transmission using electromagnetic resonance in 2007. The wireless power transmission, which is used in combination with wireless communication, is attracting attention as a technique for implementing electric power transmission in a completely wireless manner (Wireless Power Transmission Technique Developed to Light 60 W Bulb in Experiment, Nikkei Electronics, vol. 966, Dec. 3, 2007).
Furthermore, in recent years, the wireless power transmission technique is increasingly used in a wide variety of applications such as charging electric vehicles and hybrid vehicles, transmitting power to such vehicles, and charging small devices such as smart phones. Under the circumstances, techniques for safely transmitting electric power have also been developed. For example, Japanese Patent Laid-Open No. 2013-38924 discloses a technique in which a power supply device installed on the ground performs authentication and power supply with respect to a vehicle that has entered a power supply range of the power supply device. Specifically, a communication unit provided in the power supply device performs communication with the vehicle, and performs authentication to determine whether or not the vehicle is a vehicle to which electric power can be supplied. Subsequently, a power supply unit provided in the power supply device supplies electric power to the vehicle. The power supply device receives, from the vehicle, information regarding the amount of electric power received by the vehicle. If it is confirmed that the amount of electric power is equal to an amount of electric power obtained by subtracting a loss between the power supply unit and the power receiving unit, the power supply device determines that the vehicle is a vehicle that has been successfully authenticated, and continues the supply of power.
With the conventional technique described above, however, the vehicle authenticated by the communication unit may not necessarily be the same as the vehicle to which the power supply unit is supplying power. In the case where multiple power supply devices are located close to each other and thus the communication ranges of the communication units of the power supply devices overlap with each other, multiple vehicles in the power supply ranges of the power supply units of the multiple power supply devices cannot recognize to which communication unit the communication and authentication have been performed. That is, such a vehicle may be accidentally authenticated by a communication unit that is not connected to the power supply unit that can supply power in the range in which the vehicle is present. Once the power supply device starts charging in this state, the power supply device continues power supply if the amount of power supplied and the amount of power received are accidentally determined as compatible.
As a consequence, a problem occurs if a large vehicle such as a truck or a bus and a small vehicle such as a light automobile are in the power supply ranges of adjacently arranged power supply devices. That is, because the battery capacity varies according to the vehicle size, if electric power is supplied from a power supply device that is different from the intended one, the battery of the large vehicle may not be charged, or the battery of the small vehicle may be damaged. Specifically, the power supply devices usually have a communication range larger than a power supply range, and thus the vehicle authenticated through communication may not be the same as the vehicle that is present in the power supply range. Accordingly, the power supply devices need to check whether a vehicle that is in their power supply range is the same as the vehicle authenticated through communication.